A Differential Mobility Spectrometer (DMS) system typically performs a gas phase ion sample separation and analysis. It operates with an asymmetric waveform alternating between high and low field conditions, and includes the use of at least two parallel electrode plates separated by a constant sized gap. A similar, but related system that also uses asymmetric waveforms is the Field Asymmetric Waveform Ion Mobility Spectrometer (FAIMS) which typically describes the use of a cylindrical configuration which includes an inner and an outer electrode, separated by a constant sized gap. In some circumstances, both DMS and FAIMS devices have been interfaced with a mass spectrometer (MS) to take advantage of the atmospheric pressure, gas phase, and continuous ion separation capabilities of the DMS/FAIMS and the detection accuracy of the MS.
By interfacing a DMS/FAIMS with an MS, numerous areas of sample analysis, including proteomics, peptide/protein conformation, pharmacokinetic, and metabolism analysis have been enhanced. In addition to pharmaceutical and biotech applications, DMS-based analyzers have been used for trace level explosives detection and petroleum monitoring.
A DMS/FAIMS, like an ion mobility spectrometer (IMS), is considered an ion mobility based analyzer because the DMS/FAIMS/IMS separates and analyzes ions based on the mobility characteristics of the ions. In an IMS, ions are pulsed into and pass through a drift tube while being subjected to a constant electric field. The ions interact with a drift gas in the drift tube and the interactions affect the time it takes for the sample ions to pass through the drift tube, e.g., the drift time. These interactions are specific for each analyte ion of a sample, leading to an ion separation based on more than just mass/charge ratio. In contrast, in a TOF MS, there is a vacuum in the drift region of the MS and, therefore, an ion's time through the MS drift region is based on the ion's mass-to-charge ratio (m/z) in the collision-free environment of the vacuum.
A DMS/FAIMS is similar to an IMS in that the ions are separated in a drift gas. However, unlike an IMS, the DMS/FAIMS uses an asymmetric electric field waveform that is applied between at least two parallel electrodes through which the ions pass, typically, in a continuous manner. The electric field waveform typically has a high field duration at one polarity and then a low field duration at an opposite polarity. The duration of the high field and low field portions are applied such that the net voltage being applied to the DMS/FAIMS filter electrodes is zero. While certain figures and embodiments herein described pertain specifically to DMS devices, it would be appreciated by the skilled person that a FAIMS device could also be utilized to carry out a similar procedure with modification where appropriate.
FIG. 1A shows a plot 100 of one variant of the time-varying, RF, and/or asymmetric high and low voltage waveform 101 (e.g., Vrf) that can be applied to generate an asymmetric electric field. FIG. 1B shows a diagram of a DMS filter 102 where the path of an ion M+ is subjected to an asymmetric electric field resulting from the asymmetric voltage waveform 101. This ion can also be multiply charged. The ion's mobility in the asymmetric electric field indicates a net movement 103 towards the bottom electrode plate of the DMS filter 102. This example shows that, in a DMS, an ion's mobility is not constant under the influence of the low electric field compared to the high electric field. Since an ion may experience a net mobility towards one of the filter electrode plates during its travel between the plates, a compensation voltage (Vc) is applied to the filter electrodes to maintain a safe trajectory 104 for the ion through the DMS filter 102 without striking one of the filter electrodes. The ions are passed between the two filter electrodes by either being pushed through with a pressurized gas flow upstream of the filter electrodes or pulled through by a pump downstream from the filter electrodes.
In a DMS or IMS, ions are typically separated in a gas at pressures sufficient to enable collisions between sample ions and neutral drift gas molecules. The smaller the ion, the fewer collisions it will experience as it is pulled through the drift gas. Because of this, an ion's cross sectional area can affect the ion's mobility through the drift gas. As shown in FIG. 1B, an ion's mobility may vary with electric field strength. This difference in mobility may be augmented by clustering/de-clustering reactions taking place as an ion experiences the weak and strong electric fields. An ion may experience clustering with neutral molecules in the drift gas during the weak field portion of the waveform, resulting in an increased cross sectional area. During the strong field portion of the waveform, the cluster may be dissociated, reducing the ion's cross sectional area. Alternatively, differences between high and low field mobility behavior may be due to different collision dynamics due to changes that occur in ion translational energy, such as polarization effects.
The integration of a DMS with a MS can provide added selectivity that can be used for purposes such as chemical noise reduction and elimination of isobaric interferences. This general reduction of the chemical background can provide improvements in the detection limit (defined for example as 3 σ/slope of the calibration curve) for various assays. One of the key factors limiting general applicability of DMS technology with MS analysis is the reduction in instrument sensitivity that is observed upon installation of the DMS. These sensitivity reductions may occur as a result of many different phenomena, including: 1) diffusion losses in the DMS itself, 2) inefficiencies in ion transport into and out of the DMS, and 3) ion clustering. It is believed that some of the losses currently being observed with the DMS at high solvent flows are a result of sampling a “wet spray” into the DMS and subsequently filtering clusters that do not transmit at the same Vc as the unclustered parent ion. This hypothesis is supported by modeling of diffusion behavior, as well as experimental data showing improvements in the coefficient of transmission with additional heaters located in front of the DMS.
In existing DMS-MS systems, there are several approaches where desolvation or declustering are utilized including: 1) the source region where turbo heaters can be operated up to 750° C., 2) a counter-current gas flow region established by the heated curtain gas, and 3) a declustering region within the first vacuum stage where the potential difference between the inlet orifice and first vacuum lens element provides some declustering. Existing DMS-MS systems typically locate the DMS before the orifice of the MS, which results in a limitation in that ions and clusters are filtered prior to the orifice, eliminating the ability to decluster within the first vacuum stage. Elimination of this stage of declustering may result in sensitivity reduction with the DMS, with higher solvent flows being most problematic. Efforts to add additional heating and provide additional desolvation prior to the DMS have shown some improvement in sensitivity, however, they have imparted very significant challenges with respect to commercialization due to the critical importance of maintaining a constant temperature and the difficulty of monitoring temperature in close proximity to very high AC potentials. The range of assays that can exhibit detection limit improvements with the DMS is limited by the magnitude of the sensitivity reduction that is observed with the DMS device.
Improvements in resolution in the DMS can be made by adding a modifier liquid to the drift gas. Modifiers can provide selectivity by clustering with ions to varying degrees, which can shift the differential mobility properties of the resulting ions. The liquid modifiers can include a wide ranging field of solvents, including alcohols-such as 2-propanol, water, as well as hydrogen and deuterium exchange agents (such as D2O or CH3OD). However, the introduction of a modifier has often resulted in a decrease in sensitivity which leads to a loss or reduction in signal intensity. Representative examples of such a phenomenon are depicted for various samples in FIGS. 2 to 5. Additionally, stability issues have been noted when using very small flow rates for liquid modifiers.
Accordingly, there is a need to improve mobility based resolution and specificity, and to increase the applicability of DMS type analyses by providing improved sensitivity and selectivity, including for high flow analyses.